1. Field of the Invention
The invention relates to a sensor comprising a substrate on which a plurality of resistive elements is arranged in a Wheatstone bridge configuration, whereby at least one of the resistive elements demonstrates a magneto-resistive effect, the resistive elements being provided in branches which respectively connect:
(a) a current input terminal with a first voltage terminal; PA0 (b) the first voltage terminal with a current output terminal; PA0 (c) the current output terminal with a second voltage terminal; PA0 (d) the second voltage terminal with the current input terminal.
Sensors of this type may be employed inter alia:
in compasses, for detecting the terrestrial magnetic field, e.g. in automotive, aviation, maritime or personal navigation systems;
in position sensors and/or angle sensors (e.g. in automotive applications);
as field sensors in medical scanners, and as replacements for Hall probes in various other applications;
as current sensors, whereby the magnetic field produced by an electrical current is detected;
as memory cells in Magnetic Random-Access Memories (MRAMs);
as magnetic heads, which can be used to decrypt the magnetic flux emanating from a recording medium in the form of a magnetic tape, disc or card.
2. Description of the Related Art
Magneto-resistance is a phenomenon whereby the electrical resistance of a body can be influenced by magnetic flux. In particular, the electrical resistance of the body changes in a predictable manner in response to a varying magnetic flux, making such a body suitable for use as a magnetic-electric transducer in a magnetic field sensor. However, as with any resistive body, the electrical resistance of such a body can also be influenced by other environmental factors, particularly temperature. A problem in (sensitive) practical applications is thus to devise some means of differentiating between transducer signals resulting from (varying) magnetic flux and (unwanted) transducer signals emanating from other environmental sources. A popular approach is to include at least one magneto-resistive element in a Wheatstone bridge arrangement.
A sensor as specified in the opening paragraph is known, for example, from PHILIPS SEMICONDUCTORS data handbook SC17 (1995), Semiconductor Sensors, pp 33-38. The sensor therein described employs the so-called flipping technique to allow it to measure very weak magnetic fields. According to this technique, the magnetization direction in each of the Wheatstone bridge's magneto-resistive elements is periodically reversed using an external flipping field. Such periodic reversal causes a corresponding reversal of the polarity of the bridge's output signal as a function of applied magnetic field. Addition of the normal and inverted outputs allows the bridge's zero-point offset to be determined and (electronically) eliminated, such offset being principally caused by mask alignment errors during fabrication of the bridge.
The known sensor suffers from a number of disadvantages. In particular, a relatively large electrical current (of the order of about 1 ampere) is generally needed to generate the required flipping field, causing greatly increased power consumption and consequently hindering the use of batteries as a power source. In addition, the flipping technique can have a destabilizing effect on the magnetization of the magneto-resistive element(s), particularly when such elements are based on the Anisotropic Magneto-Resistance (AMR) effect; such destabilization leads to increased noise levels.